Conventionally, the two cavities of such a klystron are designed to resonate at the same ultra-high frequency, i.e. the desired operating frequency of the oscillator. To vary this operating frequency, the two cavities must be retuned simultaneously; since the resonance frequency of each cavity is determined by its dimensions, such retuning requires a structural change by a displacement of a piston or the like constituting a mobile wall portion thereof. In view of the high Q of such a resonant cavity, its tuning is a delicate operation; the properly correlated tuning of two cavities by respective pistons is therefore difficult and time-consuming. The use of a single diaphragm common to both cavities, replacing the respective tuning pistons thereof, simplifies the adjustment procedure but affects the intercavity feedback and therefore the amplitude of the driving voltage applied to the buncher gap.